Firstly they didn't do anything wrong yet
Maybe not, but they certainly threatened to do something very bad, and that is very serious, even if they didn't do it/changed their minds. You must consider the gravity of the situation when you have your own population claiming they're going to join a country in war against you.
However, sentencing them severely doesn't seem to be a method of rehabilitating the ones commiting this crime. What they appear to be doing is setting an example, and telling people that the fighting in the Middle-East is no laughing matter, and you need to consider what you are doing and saying very carefully before making such a decision.
Each of these individuals have made it clear they planned to raise arms against Allied troops and kill them. I don't know about you, but if I said something like that here, whether I were joking or not, I would be made a serious example of. If not by the Government trying to keep order, then by rampant "offended" civilians.
They're getting off easy with only a few years imprisonment, if you speak on terms of "making an example."
They've pretty much ruined themselves severely already, on being productive members of society. The man has already made headlines available for the whole world. Would you hire someone that once planned to betray his country's government just for some spiritual fulfillment?